In a cable television subscriber network of a star-net type, the television signals are usually transmitted on a broadband line from headends of the cable television system to different splitting points. The television spectrum is divided in to different frequency bands called channels on which different television programs are transmitted by means of an amplitude modulated VSB signal. Different subscribers are then connected to these splitting points in which the television channels for which the subscriber subscribes are provided, and in which other channels are jammed, in particular premium television channels, in order to prevent unauthorized viewing.
At the headend of the cable television system there is usually provided a unit for controlling the system and managing all data traffic between the headend and the splitting points, which for example can comprise an interdiction apparatus, and giving instructions to devices located in the splitting points, such as the interdiction apparatus.
U.S. Pat. No. 4,912,760 describes a cable television interdiction apparatus having means for controlling a plurality of voltage controlled oscillators for selectively jamming only unauthorized premium programming transmitted in clear from a headend to a particular subscriber.
U.S. Pat. No. 4,792,971 describes a cable television system having means for controlling viewing access to different designated television channels.
U.S. Pat. No. 5,467,397 discloses a cable television interdiction apparatus comprising a micro-processor and control means for controlling voltage controlled oscillators used for selectively jamming unauthorized premium programming transmitted from a headend to a particular subscriber.
In the above cited patents several conventional voltage controlled oscillators are used for jamming a particular part of the frequency band each. Such oscillators then cover the frequency range from a certain frequency to around twice that frequency, i.e. f.fwdarw.2f. However, if a better accuracy of the frequency control of the oscillator is desired the frequency range thereof can be lowered.
This results in that, in order to jam a television signal which ranges over a large frequency band, usually being larger than from the lowest frequency to the double frequency, e.g. from f to 4f, several jamming oscillators are required.
For example, in a conventional cable television system making use of the entire frequency range, ranging from 47 MHz to around 550 MHz, a number of different conventional voltage controlled oscillators are required only for covering the entire bandwidth. A particular oscillator jams the different channels transmitted in a particular frequency range according to some pattern or at random by means of transmitting a signal pulse for a short time for a particular channel and then hop to another frequency in order to jam another channel.
In the prior art technique, if several premium or pay television channels are located closely in frequency, there is a problem to efficiently jam these for unauthorized viewing, since the number of programs that a jamming oscillator can jam per cycle, is limited. Thus, in order to provide an efficient jamming of a program, several jammings per cycle may be required. Moreover, in some cases it is also desired to jam the sound. This of course adds additional requirements on the oscillators, often resulting in that the capacity of a particular oscillator intended to jam a certain frequency band, is insufficient.